Jiang Chun Geng’s poisoned right leg, with its suppurating wounds, hangs limply over the gray wooden bench in the medical clinic here in Dachen, a village in China’s province of Zhejiang. Twice the size of his left leg, the limb is too tender to touch during my visit. Instead, Dr. Zhu Jian Jun gently dabs the putrid wounds with an alcohol-drenched swab. Jiang’s heavily lined face tightens as Zhu wraps the fiery stump with a white bandage and unhooks an intravenous antibiotic drip. Another treatment is over.

Jiang, a 70-year-old farmer, can’t remember a time when flesh-eating ulcers didn’t cover his legs. “They never go away,” he tells me. “They just get drier. Sometimes they hurt less.” He doesn’t know for sure how he got them, but his father told him that the wounds first appeared in July 1942, soon after the Japanese army passed through his village. His entire family developed the festering sores. His mother and younger brother died in unbearable pain a decade later as the untreated, mysterious infection crept up their legs.

Jiang is one of 15 elderly Chinese men and women whom Zhu is treating in his simple village clinic for what locals label “rotten leg disease.” A definitive diagnosis is no longer possible so many decades after the initial exposure and secondary infections. But Chinese, American, and other Western physicians who have examined the survivors, documented their histories, and photographed their wounds claim that they are victims of the most gruesome biological warfare attacks in modern history.

These attacks, orchestrated by Japan’s infamous Unit 731 between 1932 and 1945, are the only documented mass use of germ weapons in modern times. Scholars say that we will never know exactly how many were killed. Sheldon H. Harris, the late American historian, estimated in a pioneering work that between 10,000 and 12,000 Chinese prisoners perished in the bloodcurdling experiments that Unit 731 performed in Japanese-occupied Manchuria. Another 300,000 to 500,000 civilians died, he wrote, as a result of Japan’s massive germ assaults on more than 70 Chinese cities and towns. China itself has disclosed no official tally. In fact, for many years, Japan’s use of biological weapons in China was largely forgotten. Only recently has a resurgent China begun to remind Japan—and the world—of the atrocities.

In America, the historical amnesia has a Cold War source, argues Jeanne Guillemin, a political analyst at the Massachusetts Institute of Technology. After World War II ended, the United States and the Soviet Union sought information about Japanese germ experimentation for their own covert bioweapons efforts. In 1947, American officials secretly agreed to grant immunity from war-crimes prosecution to the head of Japan’s program, Major General Shiro Ishii, and to 18 other biowarfare scientists in exchange for intelligence. As Harris recounted, Japanese pathologists gave U.S. officials some 8,000 slides of human tissues and blood samples taken from autopsied victims, as well as knowledge about the warheads and bombs that Japan had designed to spread its killer germs.

While several American military historians and scientists have downplayed the value of the Japanese data, a U.S. military document unearthed by Guillemin called the information “of the highest intelligence value,” though before it underwent full analysis. The Japanese program had to remain secret, another military document concluded (in diplomatic understatement), “to protect the interests of the United States and to guard against embarrassment.” Only after America and the Soviet Union (the Soviets only putatively) ended their offensive germ-weapons programs in the late 1960s and signed a treaty banning pathogens as offensive weapons in 1972 did significant amounts of evidence about the arrangement become public.

A second foreign-policy consideration also helps explain America’s deal and ensuing silence, Guillemin points out. American officials believed that prosecuting Japanese scientists for war crimes that the widely revered emperor himself might have authorized would jeopardize Washington’s effort to rebuild Japan as a stable democracy, “crucial to offsetting the Soviet presence in Asia and the rise of Communist China,” she writes. Indeed, General Douglas MacArthur personally intervened to protect Japanese scientists who had worked on the program from prosecution.

Though President Clinton ordered U.S. government agencies in 2000 to make more than 100,000 pages of official documents on the program available to scholars, key questions remain—for instance, which American official authorized the arrangement with the Japanese biowarriors. The answers are unlikely to be found in Japan, whose own archives on the initiative remain largely closed to public scrutiny.

As for China itself, historically it has vacillated between stoking anti-Japanese nationalism and trying to dampen it, depending on its political needs. For instance, China once estimated that 10 million of its citizens died during Japan’s traumatizing occupation of eastern China and Manchuria—the “Anti-Japanese War of 1931 Through 1945,” as China calls it. Then, after the Tiananmen Square crisis, China raised the estimate to 35 million, seeking to divert attention from domestic discontent. But it’s surely the case, observes Susan L. Shirk, a former State Department official and the author of China, Fragile Superpower, that almost every Chinese family suffered under the onslaught.

Only now that China has emerged as Asia’s leading power has it begun to highlight Japan’s biological crimes against it. The focal point of this effort is the Unit 731 Museum, just south of the city of Harbin in Manchuria. Built on the ruins of a cluster of ten villages known as Ping Fan, the museum occupies what was once the headquarters of Japan’s germ empire. Constructed by Chinese slave laborers in 1936, Unit 731, whose Orwellian cover name was the “Epidemic Prevention and Water Supply Unit,” was a vast laboratory complex of 3.7 square miles with more than 70 buildings—laboratories, officers’ housing, a Buddhist temple, an airfield and railway station, three crematoria to dispose of experimentation victims, a prison, a power plant, and even a brothel to service the 3,000 Japanese scientists and guards who lived and worked there during its peak. Behind the complex’s high, heavily guarded walls, Major General Ishii’s scientists experimented on Chinese, Americans, Koreans, Mongolians, Russians, and others with some of the world’s deadliest germs.

The museum’s main exhibition hall, located in Unit 731’s administration building, describes Ishii’s program and takes visitors through the procedures that his medical team followed in processing doomed inmates. Maruta, or “logs,” as the Japanese scientists dubbed their victims, would be registered, given numbers, and later dragged from their cells through underground tunnels into the testing labs at the compound’s center. Here, Sheldon Harris reported, they would have to eat food laced with one of 31 germs—anthrax-filled chocolate, plague-treated cookies, typhus-infected beer—or be injected directly with deadly pathogens to determine the minimal dose required to sicken or kill them. The “logs” usually lasted only a few weeks. Some were “sacrificed” after unit officials deemed them no longer fit for scientific study. Only a few survived for as long as six months, Unit 731 documents show.

Room Six of the museum contains a display of the high- and low-altitude bombs that the Japanese developed to disseminate their lethal cargo. One bomb included a porcelain cylinder to prevent germs from destruction on impact. Another was called “Mother and Daughters”—the Mother part of the weapon coming with a radio device that would detonate a cluster of germ-loaded Daughter bombs. That weapon proved too expensive for mass production, Harris observed. But other types of anthrax-, plague-, and bacteria-filled bombs were tested on hundreds of prisoners, who were tied to stakes in the ground and died in agony.

Room Nine displays fragmentary evidence of Japan’s “fascistic guilt which cannot be denied,” as the museum puts it: a wooden saw for dismembering human remains and racks for hanging entrails fresh from autopsies, some performed while the victims remained alive to ensure that death didn’t alter the disease’s impact on the organ or bones under study. I recalled interviewing an elderly Japanese soldier several years earlier who told me that he had performed vivisections, without anesthetic, on naked prisoners. Describing in almost a whisper his revulsion the first time he picked up his scalpel when ordered to do so, he said that he eventually grew accustomed to the “procedure.” But his anguish suggested otherwise.

The museum’s holdings are necessarily limited. When the Japanese had to flee Ping Fan in 1945, they attempted to destroy all traces of their crimes against humanity—as the Nuremberg tribunal called similar experiments performed by Nazi doctors in Europe—by killing the last 400 prisoners, setting infected animals free, and blowing up the complex.

Dr. Jin Cheng Ming, the museum’s director, is cautious about death-toll estimates. The museum has been able to identify only 277 of the thousands of prisoners who came here, he said, mostly from the records kept by the Japanese transportation units that delivered them. But his estimates, which draw on provincial records and archives, are comparable with Harris’s. In his Chinese-language book on the program, published in 2008, Jin concluded that so much of the widely dispersed program was secret that the death toll might remain uncertain even if Japan cooperated. “Maybe the Japanese know exact numbers,” he says. “Or maybe your government does, since Japan shared so many records with you.” In 2006, he adds, the U.S. gave China access to about 2,000 more pages of declassified records, but most were from late in the program. He and his staff have made 17 trips to Japan to collect oral evidence from former members of Unit 731. “It’s hard because Japan is not sympathetic,” he says. But part of the museum’s mission is to videotape such testimonies for its collection, which now features over 200 hours of them.

After opening in 1985 with a one-room exhibit and a staff of three, the museum languished for years. Few foreigners or even Chinese came here. Today, though, it bustles with activity and is undergoing a vast expansion. Thirty-one sites are being excavated or renovated in and around the museum, which received more than 6 million visitors last year, about half of them Chinese schoolchildren. The Chinese government has tripled the museum’s budget, and its staff has grown to 40.

The feverish activity is part of Jin’s campaign to have the United Nations Educational, Scientific, and Cultural Organization designate his museum a cultural-heritage monument—joining another infamous death complex, Poland’s Auschwitz. UNESCO sent a team to the museum late last summer to examine the site, and Jin is optimistic that the organization will eventually add his museum to its list. “While we still have many questions, we have so much more evidence than we had before,” he tells me. “People must know and face their history. And the Japanese government should be responsible for its actions.”

As horrifying as the experiments performed at Unit 731’s headquarters were, they pale beside the horrors that Japan inflicted on the Chinese population at large. As we travel by bullet train from Shanghai deep into the province of Zhejiang, activist Wang Xuan tells me that the worst attacks occurred between 1940 and 1943, when the Japanese military struck dozens of Chinese cities and towns with pathogens that triggered recurring plague epidemics and killed hundreds of thousands.

The methods were brutal. Army trucks dumped gallons of deadly germs alongside roads and railway lines linking Chinese towns so that infections would spread from town to town; planes dropped porcelain bombs containing infected fleas on dozens of villages, causing devastating outbreaks of bubonic plague. The Japanese laced more than 1,000 wells in the area of Harbin with typhoid bacilli. They also inserted typhus into bottles of lemonade that children loved to drink in the summer, Harris reported. In Nanking, they distributed anthrax-filled chocolate and cake to hungry children. The Japanese discovered that packing fountain pens and walking sticks with deadly germs was a particularly effective way of secretly disseminating them. In 1940, Major General Ishii sent a train carrying 70 kilograms of typhus bacterium, 50 kilograms of cholera germs, and 5 kilograms of plague-infected fleas to the city of Hangzhou, a holiday resort favored by Shanghai’s wealthy. From there, the germs were dumped into ponds and reservoirs and spread by aerial spraying, contaminating all life in fields of wheat and millet during the harvest.

As our train pulls into the station, Wang Xuan tells me that near here, over the small town of Quzhou, a Japanese plane dropped plague-infected fleas in October 1940. The first victims died a few days later. By 1948, more than 5,000 people had perished. Prior to that outbreak, no case of plague had ever been reported in the town, says Dr. Qiu Ming Xuan, former chief of the Quzhou Center for Disease Control.

That fact proved important in 1997, Wang explains, when she helped organize a lawsuit against Japan for spreading bubonic plague and other fatal diseases across China. Still, the town’s plague-free earlier history—as well as oral histories from elderly residents of Chongshan, a town near Quzhou, and epidemiological data from local and provincial files—might not have been sufficient to persuade the court to side with the Chinese plaintiffs, since biological samples from the 1940 attack were no longer available. But Chongshan’s survivors submitted to the court an original document: the working diary of a senior Japanese military officer, then stationed in Nanjing, responsible for contacts between his forces and Unit 731. The diary mentioned a plague attack on Quzhou on October 4, 1940—a date that coincided perfectly with the plague’s spread from Quzhou to Chongshan.

In 2002, the Japanese court finally ruled that Japan had indeed used biological weapons against China—the first such official acknowledgment. But the court also ruled that the Chinese were not entitled to compensation under international law. Nor did Tokyo itself acknowledge having used germ weapons during the war or apologize to the Chinese victims. Since then, a succession of Japanese governments has remained silent about the crimes.

Sufferers from “rotten leg disease” have no comparable diary entry or official Japanese source that could prove to a court that their suffering was related to Japan’s germ-weapons attacks. Indeed, even some Japanese researchers sympathetic to China challenge the claim that Japan’s biowarfare caused the disease. Keiichi Tsuneishi, a distinguished Japanese historian who helped uncover Japan’s atrocities in China, says that a bacterium native to the region could have been responsible, and he estimates that pathogens killed no more than 1,000 Chinese in the province of Zhejiang.

Wang Xuan responds that Tsuneishi is neither a biologist nor a physician and that he has never visited the affected regions. Refusing to yield to such challengers, she recruited four university students last summer to interview “rotten leg” survivors of the 1942 outbreak and to locate more victims. According to both Qiu and the students, in the villages surrounding the small city of Jinhua in central Zhejiang, 5 percent of the residents born before 1942 described having developed raw, festering wounds on their legs similar to those of the patients I saw.

In Blood-Weeping Accusations, a book in Chinese and English published by the Chinese Communist Party in 2005, some Chinese researchers conclude that anthrax caused the wounds. But at least two American physicians—Martin Furmanski, a pathologist and physician who wrote an essay in the book, and Michael Franzblau, a retired professor of dermatology at the University of California—believe that the culprit is Burkholderia mallei, or glanders. A horrendous skin disease that usually strikes horses, donkeys, and mules, glanders in humans is “exquisitely painful,” Furmanski says. Left untreated, it causes deep wounds that will not heal unless treated with antibiotics. “Pustules come up from below, point, break, and ooze a pussy fluid,” Furmanski explains. “Untreated, they can stay for months or even years. Only rarely do they heal. And when they occasionally do, you get a terrible scar.” Cutaneous anthrax, by contrast, starts as a lesion on the skin that blackens and swells. “You look like the Michelin man,” he says—but when it heals, it doesn’t leave a scar.

Furmanski became convinced that glanders, not anthrax, was the cause of “rotten leg disease” after he and Franzblau toured Quzhou with Wang Xuan in 2002. While taking a sample from a wound 60 years after the initial exposure probably would not reveal the identity of the original infectious agent, it might shed light on the nature of the infection or how best to treat it, he notes. But Furmanski says that he didn’t try to culture a sample and take it back to America because he hadn’t requested China’s permission. Moreover, the shipment of glanders is restricted in America, given its potential use as a biological weapon, he adds. Indeed, glanders remains a research subject at the U.S. Army Medical Research Institute for Infectious Diseases, the military’s premier biodefense lab, at Fort Detrick, Maryland. “Legally, I couldn’t bring it back into the United States,” Furmanski says.

Working closely with Wang, Franzblau has tried for years to introduce a resolution at World Medical Association meetings calling upon doctors to ask Japan to “officially repudiate Unit 731” and to explain “why physicians employed in Unit 731 have never been prosecuted for murder and crimes against humanity.” Each year, his resolution has gone nowhere. “There has never even been a debate,” he complains. The Japanese Medical Association has also remained silent, perhaps because one former president of the JMA was a Unit 731 staff member, as were former officials in many prestigious Japanese organizations.

Wang has raised tens of thousands of dollars to provide survivors the medical care they so desperately need. The Chinese government has done little to care for them, she says. Nor has any money come from Japanese organizations or individuals, unsurprisingly. And she has another fear: “I am worried that when these people die, the memory of what Japan did in China will die with them.”

According to Jan van Aken, a German writer, activist, and now Bundestag member who accompanied Wang on a tour of the region in 2006, “almost the entire microbiology elite of wartime Japan may have been involved in one way or the other in the heinous crimes of Unit 731.” In postwar Germany, by contrast, doctors who participated in similar experiments were exposed and convicted of crimes against humanity. “The time has come for Japan to appoint an independent, international commission, to investigate and publicly document, without restraints, the involvement of former members of their profession,” says van Aken. “This was the most intensive use of biological weapons in modern history. Yet Japan remains silent. And so do most of us.”

Judith Miller is a contributing editor of City Journal, an adjunct fellow at the Manhattan Institute, and a FOX News contributor.